The basic goal of a typical Posterior Lumbar Interbody Fusion (“PLIF”) procedure is to remove a problematic disc, and insert a prosthetic fusion device (such as a cage or a mesh) into the empty disk space created by removal of the natural disc. However, the pathophysiology of an intervertebral disc is such that the disc space in which the disc sits is typically collapsed (e.g., 30-50% of the time) prior to disc removal. Once the natural disc is removed (but prior to implant insertion), the annulus and soft tissue surrounding the disk space tend to force the adjacent vertebrae to come even closer together, thereby increasing the extent of disk space collapse. Since one goal of the surgery is to restore the patient's anatomic disc space to the extent possible, there is a need to spread apart these collapsed vertebrae. In one conventional PLIF procedure, a flat Spreader (which resembles a butter knife) is inserted with a horizontal disposition into the collapsed disk space and then rotated 90 degrees to vertically distract the adjacent vertebrae and restore the patient's physiologic disc space. The distracted endplates are essentially parallel to each other after the Spreader is rotated.
In addition, it is further desirable that the implanted device be secure within the disk space. However, since the geometry of the disk space varies from patient to patient, and the implants are typically manufactured in only a few shapes, the implants do not typically fit congruently into the distracted disk space. Accordingly, in one conventional PLIF procedure, congruence between rectangular implant shapes and the distracted disc space height is achieved by forming rectangular channels of known dimension in the adjacent endplates.
However, the current PLIF procedure for forming substantially rectangular channels in the endplates is a time-consuming three-step process. After the full discectomy and careful distraction of the disk space by the Spreaders described above, the surgeon must first insert a Reamer into the disk space and rotate it to create rounded grooves in both the superior and inferior endplates. In a second step, the surgeon then must insert the leading edge of a Pilot Broach into the disc space and axially impacts it to create a rectangular channel on the posterior side of the endplates. In the third step, the surgeon inserts a Finish Broach into the disc space and axially impact it to complete the anterior portion of the rectangular channel. Further details of the three-step Reamer—Pilot Broach—Finish Broach procedure and conventional instrument shapes can be found in a Brantigan et al. “Posterior Lumbar Interbody Fusion Techniques Using the Variable Screw Placement Spinal Fixation System” 6(1) in Spine: State of the Art Reviews. January 1992, pp. 175-200.
The Pilot Broach noted above has a body portion, a rectangular shaver portion extending distally from the body portion, and a cylindrical spreader portion extending from the shaver. The larger axial silhouette of the shaver portion relative to the cylindrical spreader portion defines shaving corners. The leading edge of the cylindrical spreader portion has a flat. Cylindrical spreader portion also includes an upper surface and a lower surface each of which bear upon the endplates. A crown is situated at the proximal end of the Pilot Broach and is used to engage a complimentary engagement connection.
Although the PLIF procedure using the Pilot Broach has been popular with surgeons, there have also been requests for improved instrumentation. In particular, surgeons have requested a quicker, easier 1-step method of preparing the rectangular channel in the endplates prior to implantation.
FIGS. 11b and 11c of U.S. Pat. No. 6,096,038 (“Michelson”) discloses a combined distractor-cutter having a distracting portion (102 in FIG. 11b and 260 in FIG. 11c) and a rotary shaving portion (270 in FIG. 11b and 250 in FIG. 11c). The distracting portion distracts the vertebral bodies while the rotary cutting portion prepares a bore shaped for the insertion of the threaded circular fusion cage.
Since each of these devices uses a rotary cutting device to prepare the endplates, the channels formed thereby are not substantially rectangular. In addition, cannulated technology (which protects the internal organs from the rotary cutting devices) is often needed when drilling devices are used, thereby increasing the complexity of the procedure.
FIGS. 25 and 25a-d of U.S. Pat. No. 6,174,311 (“Branch”) disclose a chisel having i) distracting portions 272, 273 for centering the chisel between the vertebrae and ii) upper and lower shavers 268, 270 for forming a rectangular channel in the distracted vertebrae. Arms 267 and 269 define a cavity 276 for receipt of bone chips and shaving debris. FIGS. 37a-c and 38 of Branch disclose a second chisel substantially similar to the chisel of FIG. 25.
One weakness of the Branch chisel lies in the disposition of both arms 267,269 and non-cutting edges 272, 273 at the lateral edges of the device. Because these arms and edges are disposed laterally, their effective widths essentially equal the entire width of the cutting edges, and thereby interrupt the surgeon's sightlines into the disk space.
U.S. Pat. No. 5,722,977 (“Wilhelmy”) discloses a combination osteotome and spacer guide. In use, as in FIG. 18, the spacer guide 8 is first inserted into the disc space 9. Next, the hollow osteotome 9 is slid over the outer dimension of the spacer guide 8 to its appropriate position. Lastly, driving head 49 of the osteotome is impacted by a mallet to drive the osteotome over the guide and into the vertebral bodies, and to cut and remove the desired amounts of bone.
One weakness of the Wilhelmy design lies in its need to slide the osteotome over the spacer guide in order to form the rectangular channel. Such sliding requires the maintence of close tolerances between the outer surface of the guide and the inner surface of the osetotome. This close tolerance may degrade with continued use. In addition, Wilhelmy teaches using separate osteotome and spacer guide instruments, thereby increasing complexity and cost. Another weakness of the Wilhelmy design lies in the relatively large width of the spacer. Since the width of the spacer must essentially equal the width of the osteotome to provide accurate cutting, the width of the spacer guide must be as large as possible. Accordingly, the surgeon's sightlines are interrupted by the device.
U.S. Pat. No. 4,697,586 (“Gazelle I”) discloses a chisel having a spreader portion and a chisel configured to slide over the spreader. The surgeon using the Gazelle I device first inserts the spreader into the intervertebral space. Next, the chisel portion of the device is slid along the outer surface of the spreader and its shaving portions cut rectangular channels into the endplates.
A publicly used device (“Gazelle II”) is somewhat similar to the Gazelle I device. Its main difference with Gazelle I is that the spreader of the Gazelle II device is rotatable. The surgeon using the Gazelle II device inserts the spreader into the intervertebral space and then rotates it 90 degrees to distract the disc space. Next, the chisel portion of the device is slid along the outer surface of the rotated spreader and its shaving portions cut rectangular channels into the endplates.
Like Wilhelmy, the Gazelle I and II devices require sliding the shaving portions over the distractor portion. In addition, the box nature of the chisel obscures sightlines. Lastly, the spreader has a height to width ratio of about 3:2, and so is is not relatively thin.
U.S. Pat. No. 4,736,738 (“Lipovsek”) discloses a shaving instrument for performing posterior lumbar interbody fusion, the instrument comprising a shaft adapted to be inserted into the intervertebral space and a chisel adapted to be slidably received within the shaft.
In sum, prior art procedures and devices used for endplate preparation suffer from:                a) the need to use multiple devices in multiple steps;        b) the need to slide a chisel over or through a spacer guide; and        c) the interruption of surgeon sightlines into the disc space.        